Once, the old formula for the North Atlantic alliance was: keep the Germans down, the Americans in and the Russians out. Today virtually every part of that has changed. Yet in the 50 years since Nato's birth the relationship between London and Washington, which was crucial to the flourishing of the alliance then, has remained as hale and hearty as ever - even though the current conflict in the Balkans has produced some of the biggest-ever risks for the main member nations.

In 1949, Truman was in the White House and Attlee in Downing Street. Ernest Bevin, the tough-talking trade unionist turned Foreign Secretary, was pivotal in getting both the North Atlantic Treaty Organisation and the Marshall Plan under way. Since then, Labour has been through some twists and turns in its attitudes towards the US, going as far as wanting unilateral disarmament and the end of Nato at times, but now it is back where it started - as a firm supporter of the US and a strong defence.

George Robertson, the first Labour Defence Minister for nearly two decades, far from scrapping weapons systems, has been responsible for the first firing of British cruise missiles and the ordering of new aircraft carriers. William Cohen, the US Defense Secretary, looks at him in a particularly avuncular way.

The war in Kosovo has underlined not just the political closeness of the two Anglo-Saxon capitals, but the role of the Brits in general: from Jamie Shea in Brussels to the various British correspondents for CNN in the Balkans, this sometimes appears to be a British operation, albeit one largely carried out by the US.

The relationship between the two leaders helps to make the link between London and Washington very close. Mr Blair and Mr Clinton speak once a day, and on subjects that range much further than Kosovo. Mr Blair is widely admired in the US, and one former senior Clinton advisor likens him to John Kennedy: he has genuine star appeal in the US, even if London has long tired of his boyish charm.

About half the British cabinet has been through Washington in the last few days: Messrs Blair, Robertson, and Cook for the Nato summit, and Gordon Brown and Clare Short for the IMF/World Bank meetings. But various other less formal events have also drawn the Labour establishment through the city. On Tuesday night, for instance, Helena Kennedy QC and Peter Mandelson were brushing shoulders at a smart drinks party held by David Blagbrough, the head of the British Council in the US; on Wednesday, Anthony Giddens was delivering a Reith lecture; and on Friday, Cherie Blair and Hillary Clinton were co-hosting a drinks party at the Residence.

The special relationship is one of the most tedious and overused cliches in British journalism, whether in the service of criticism ("Special relationship in tatters as ...") or unctuous praise ("Special relationship boost as ..."). The truth is that most of the time, Britain and America have a stronger relationship than most other countries because of history; sometimes they don't. The relationship is far more important for one side than the other, and most of the time you don't need to guess very hard to work out which country is the demandeur.

But one of the reasons why the relationship is oddly strong at the moment is that the boot is, to some degree, on the other foot. Mr Blair was publicly and obviously supportive of Mr Clinton at a time when it looked possible that he would follow Richard Milhous Nixon through the back door of the White House. He was right behind the US over Iraq, and Britain was the only country to use military force alongside America. And he has not only backed the US over air strikes in Kosovo, he has taken British aircraft, ships, submarines and (for the first time ever) cruise missiles with him.

Indeed, on Kosovo, the British have been firmer than the Americans, if anything. They have broached the idea of ground troops when it is still virtually taboo in official Washington. Mr Blair has lined up plenty of media events this weekend, with the aim of underlining the solidarity of (at least his bit of) the alliance. And Mr Cook was publicly supportive of Madeleine Albright, the US Secretary of State, who has been widely fingered in the US press as responsible for underestimating Slobodan Milosevic.

Defence has always been one of the strongest strands in the rope that ties London to Washington, and it is getting stronger. Not only are the two capitals thinking very closely together at the moment; British Aerospace is preparing to complete its purchase of GEC's defence interests, which will make it one of the largest defence contractors to the Pentagon.

In the process of intervening in Kosovo, Mr Blair has come close to squaring a difficult circle. In general, pro-American Brits have trouble with Europe - Margaret Thatcher and Harold Macmillan in particular. But Mr Blair has also won brownie points in Brussels, Paris and Bonn by spearheading the idea of a European defence identity and showing what it might mean. Ahead of the summit, the British had quietly sat on what they saw as some of America's more outre ideas for turning Nato into a world policeman, while prodding some other Europeans towards greater activism.

For the moment, all this is fine. But as last year's failed attacks on Iraq showed, it can easily crumble. Then, Britain was seen as America's poodle; it is again, by some, as it follows the US into Kosovo. If things start to go wrong and the Europeans and Americans start to drift apart, then Mr Blair will have to choose sides, which could be very painful. George Robertson has frequently said that if he couldn't ride two horses, then he shouldn't be in the job. But if they start moving in different directions, he may find life a little less comfortable.